I don't see any trauma on his face/lips. Has this fish had a history of slamming into the glass hard? Not that it can't happin I'm just not sure that's what the problem is unless of course you saw him do it. Some issues can develpoe over night Most animals fish especially are very good at masking symptoms until they are advanced to aviod oredation so untill the issue becomes sever in some cases they seem normal and all of a sudden they have and issue, it's not all that uncommon depending on what's wrong.